1. The Field of the Invention
The field of the present invention generally relates to regulated electrical power supplies. More specifically, the present invention relates to electrical circuits for providing constant current regulation as well as overvoltage limitation and which are particularly well adapted for use as a battery charger.
2. The Background Art
Nearly all modern electronic devices require what is generally termed a "power supply" in order to conveniently operate the device from available sources of electricity. The most common of these is the AC power line. It is the function of a power supply to convert the alternating current found on the power line, which is often at potentials which range between 100 and 240 volts rms, to a rectified voltage generally in the range from 6 to 24 volts. Thus, the functions of a power supply are the transformation of the high power line voltages to a lower voltage and rectification of the current flow. It is often the case that a conventional power supply is used to charge what are referred to as a secondary cell, i.e., a rechargeable battery.
Generally, a power supply contains a transformer and a rectifier circuit. Additionally, a capacitor, as well as other components, may also be used in order to provide filtering to reduce the ripple present in the output voltage. While such a simple power supply may properly power some electronic circuits and devices, for optimum performance it is often necessary that the power supply be regulated. This is also true for secondary electrochemical cells.
The term "regulated" refers to the characteristic of a power supply whereby under varying load conditions (e.g., varying resistance) either the voltage or the current through the load will remain constant. Voltage regulation or current regulation is provided by an electrical circuit which is incorporated into the power supply in addition to the transformer, rectifiers, and any additional filtering components. It is nearly always the case that any electrical circuit acting as a load will perform more reliably and consistently if power is provided to it by a regulated power supply. The use of regulated power supplies also allows maximum performance from secondary electrochemical cells, groups of which are referred to as batteries. Since a battery charger is merely a specialized form of a power supply, the two terms will be used interchangeably throughout this disclosure.
The term "secondary cell" generally refers to an electrochemical cell which produces a voltage across its terminals and is capable of being recharged to its original state after being discharged. The operation of a secondary cell is in contrast to that of a primary cell, which must be discarded after it becomes discharged. It is uncommon, however, to use a single secondary cell. Rather, a plurality of secondary cells are connected in series to form a battery. For convenience, rather than referring to a secondary cell, the terms "rechargeable battery" or just "battery" will be used herein. Use of the term "battery" is intended to encompass both single secondary cells and rechargeable batteries.
Regulated power supplies and regulated battery chargers may be designed to have several different attributes. The most common of these are constant voltage and constant current. The constant voltage regulated power supply is designed to provide a constant voltage, regardless of the amount of current that is drawn or "sinked," by the load, which in the case of a charger is a battery. Alternatively, a constant current regulated power supply is designed to provide a constant current regardless of the voltage required in order to maintain the constant current flow. It will be appreciated that the actual performance of regulated power supplies are far from the ideal.
For example, it is often impractical to design and construct a constant voltage power supply which is capable of delivering "unlimited" current. Similarly, it is impractical, as well as impossible in some cases, to design and construct a constant current regulated power supply which will maintain a constant current through any load. Furthermore, some power supplies are designed to operate as a constant current supplies under some conditions and constant voltage supplies under others.
It is the application that substantially determines the type of regulation desired. A transistorized amplifier generally requires a constant voltage in order to operate properly. Alternatively, the charging of batteries is best carried out with a constant current. Rechargeable batteries are finding expanded applications in many industrial, scientific, medical, and consumer situations. Thus, providing efficient chargers is becoming more important as more applications for rechargeable batteries are found.
One of the most common examples of a rechargeable battery is the lead-acid battery used to power the starter in nearly all automobiles. A particular variant of this class of batteries is the so-called "gel-cell" battery in which the electrolyte is a highly viscous gel and the case is sealed. Since the lead-acid battery is impractical in many circumstances, other types of rechargeable batteries have been developed, including what are commonly referred to as nickel cadmium batteries. Modern nickel cadmium batteries have a useful lifetime extending through hundreds, or even thousands, of charge-and-discharge cycles. The popularity of nickel cadmium, or Ni-Cad, batteries is increasing, as they find more and more applications in many different fields.
As stated earlier, a constant current power supply is most useful for recharging batteries. This will be appreciated by understanding that the rechargeable battery is an electrochemical device. The rate of electrochemical reaction, assuming constant temperature, s generally current dependent. Thus, the material liberated in discharging an electrochemical cell (e.g., rechargeable battery) is directly proportional to the quantity of current circulated through the cell. In other words, for each ampere of current supplied by the cell during the discharge process, a calculable mass of material is liberated. This electrochemical reaction is reversed during the recharging process. For each ampere of current circulated through a cell in an ideal rechargeable battery, a certain mass of material will be deposited, until all of the material liberated during the discharge process has been recombined and the rechargeable battery is fully charged.
Importantly, the voltage found across the terminals of a battery is not an accurate indication of the state of charge of the battery. Interestingly, one of the desirable properties of "gel-cell" as well as Ni-Cad batteries is that the terminal voltage remains constant until the capacity of the battery is nearly exhausted. The same is true during the recharging process. The terminal voltage rapidly rises to a nominal voltage even through the battery may have only been recharged to ten percent of capacity.
Thus, monitoring the voltage found at the battery terminals is not an accurate indication of the state of charge of the battery. An excessively high terminal voltage, however, is indicative of an "overcharged" condition. Furthermore, the internal resistance of the battery to the "charging current" will vary greatly during the recharge process, generally being lowest when the battery is completely discharged and highest when the battery has reached its maximum charge.
Thus, if it is desired to recharge a battery within for example, four hours, the application of a constant voltage will not assure that full recharge takes place within the desired time. This is due to the fact that the internal resistance of the cell varies during recharging, and the amount of current passing through the battery will also vary. The application of whatever voltage is necessary to maintain a constant current, will on the other hand allow the battery to be predictably fully recharged within a desired time. Thus, in order to reliably recharge a battery within a desired amount of time, a battery charger should be regulated to provide a constant current.
There exist two principal methods of implementing power supply regulation: dissipative regulation and switching regulation.
In dissipative-type regulators, an electronic device, such as a transistor or vacuum tube, is operated linearly to control the flow of current through it, and thus also the voltage drop across it. Modern power supplies generally use transistors. The transistor may be configured in a series pass arrangement (where all the current flowing to the load passes through the transistor) or a shunt arrangement (where the current is shunted through the transistor rather than being allowed to flow through the load). The linear operation of the transistor allows very accurate regulation since the current or voltage supply to the load may be varied in very small increments.
It is important to note that the dissipative-type regulators inherently operate at low efficiencies. The following example will illustrate the low efficiency of the dissipative-type regulators. Assume that a "series pass" transistor is controlling the current to the load at a constant level. Also assume that a constant current of one ampere is flowing through the series pass transistor and the load and that the voltage drop across the load is 35 volts. If there is a voltage drop of two volts across the series pass transistor, the transistor would be required to dissipate a mere two watts of power and heat.
If on the other hand, the voltage required to maintain a current of one ampere diminishes to 20 volts, in order to maintain the current at a constant one ampere, the series pass transistor would be required to experience a drop of 17 volts. Thus the transistor should be capable of dissipating 17 watts of power and heat. The seriousness of this situation is dramatic where it is known that wide variations in voltage will be experienced while constant current is to be maintained.
In situations where wide voltage swings will be experienced, such as when charging batteries, it is necessary to set the operating point of the series pass transistor at approximately the middle of the expected voltage range. Thus, dissipative-type regulators inherently operate at efficiencies of 50% to a maximum 60% and are particularly inefficient when operating at the extremes of the voltage ranges, depending upon the particular type of circuit used in the regulator.
While the efficiency of electronic circuits is always a concern to the circuit designer and builder, in many applications the power or heat dissipated by a regulator is of greater concern. For example, in the medical arts, it is common to incorporate rechargeable batteries in medical devices, such as infusion pumps. Inclusion of rechargeable batteries in medical devices allows the devices to continue to provide life support functions even when AC power is removed. The use of rechargeable batteries and built-in rechargers allows the battery to be maintained in a fully charged condition and thus to be available for use at any time without the need for constant operator surveillance.
Dissipative-type regulators have a serious drawback in such applications. It is generally desirable to make such medical devices as small as possible. Furthermore, it is also desirable to enclose such devices in sealed housings without any ventilation.
As is well known, increases in component temperature cause dramatic increases in the failure rates of those components. For example, it has been found that for every 10.degree. centigrade rise in temperature, components commonly found in medical devices experience a twofold increase in their failure rates. This effect of heat generated by dissipative-type regulators is an especially serious consequence in medical devices where component failure can be potentially life threatening.
In contrast to the operation of dissipative-type regulators are the switching-type regulators. In switching-type regulators the linearly operating series pass element found in dissipative-type regulators is replaced by a switching device. Such switching devices should have the characteristics of an ideal switch, that is, infinite resistance when turned off and zero resistance when closed, or turned on. Modern transistors provide very good approximations of these ideal characteristics. When used as constant current regulators, the amount of time that the switching device is on constitutes its duty cycle and determines the average current through the load.
Since switching devices, such as transistors, SCRs, triacs, or similar devices, are either on or off, very little power loss is experienced. According to Ohm's law, power generated equals I.sup.2 R. When the switch is conducting and the resistance is zero; when it is not conducting the current is zero. Thus power dissipation will in theory be zero. While switching-type regulators often require more components than their dissipative-type counterparts, it is still possible to realize considerable savings in volume and weight with them, since much smaller and lighter weight components can be used. In particular, large filter capacitors are not needed.
Thus, the inherently higher efficiency, and lower heat generation, as well as the savings in volume and weight, make switching-type regulators compatible for use in medical device applications, such as described above. Switching-type regulators, however, inherently have other drawbacks.
First, switching-type regulators have much less precise regulation than their dissipative-type counterparts. In order to increase the precision of the regulation, it is necessary to increase the switching rate. Increasing the switching rate, however, leads to a second drawback. As the switching rate is increased to improve regulation precision, switching transients are produced a very high and higher frequencies. Thus, even reasonably precise switching-type regulators can produce an excessive amount of radio frequency interference. Unless deliberate precautions are taken to reduce the radiation of radio frequency interference, severe interference with the operation of surrounding devices can occur.
By an understanding of the foregoing, it will be appreciated that it would be an advance in the art to provide a regulated battery charger which maintains a high efficiency over a widely varying range of voltages. It would also be an advance in the art to provide a battery charger which generates very little heat and thus is suitable for use in enclosures lacking ventilation. Still further, it would be an advance in the art to provide a regulated battery charger with the above attributes which requires relatively few components and which when assembled occupies a very small volume of space.
It would be yet another advance in the art to provide a regulated battery charger which combines the above advantages and which radiates an inconsequential amount of radio frequency interference. Another advance in the art would be to provide a regulated power supply incorporating all of the above listed attributes which exhibits regulation precise enough and current carrying capacities high enough to suit it for use as a battery charger.
These and other advantages provided by the present invention over the devices found in the art will become more fully apparent with an understanding of the present invention gained by an examination of the following description of the preferred embodiment.